Classifications

• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02K—DYNAMO-ELECTRIC MACHINES
  • H02K26/00—Machines adapted to function as torque motors, i.e. to exert a torque when stalled

Definitions

• This invention relates to electric motors comprising a permanent magnet field and a wound armature.
• the invention is particularly applicable to a direct drive DC motor, for a servo-actuator, which may be directly attached to the load.
• a direct drive DC motor for a servo-actuator, which may be directly attached to the load.
• Such a motor may have a wound armature and a permanent magnet field which acting together convert electrical currents directly into torque to maintain desired accuracy in a positioning or speed control system. If continuous rotation is required commutation may be necessary and this can be provided by semi-conductor switching with timing provided by a magnetic sensor mounted on the rotor.
• the invention is however particularly applicable for a motor capable of a limited angle of rotation only, for example plus and minus 20o. In this case torque is proportional to the coil current and also varies approximately sinusoidally with angle of rotation.
• the slots which are provided so that the windings may be inserted provide an interruption in the magnetic circuit so that as the rotor turns (even when the coils are not energised) the changes of reluctance introduce an alternating torque which may be referred to as a cogging torque, which is superposed upon the desired torque sometimes aiding it and sometimes opposing it.
• This cogging torque represents a disturbing load which has to be overcome by the applied torque arising from current in the coils.
• the number of such slots provided for the windings is not necessarily related to the number of poles produced by the windings, although the number is often an integral multiple of the number of poles.
• the slots are usually equally spaced around the motor.
• an electric motor comprises two or more units each including a permanent magnet field device mounted to rotate relatively to a wound armature having coils producing the same number of poles as the field device but located in a second number of equally spaced slots, the units being mounted with their permanent field devices mechanically connected together and their armatures mechanically connected together in such relative positions that the relative positions of the armature slots in relation to the poles of the permanent magnet field device of different motor units differ by an angle substantially equal to tha slot pitch divided by the number of motor units so that their cogging torques substantially cancel out.
• the relative positions of the parts will generally be such that the relative positions at which different units exert maximum torque (at a given current) differ by an angle equal to the. slot pitch divided by the number of units, so that their cogging torques cancel out.
• the second number of slots may usually be a multiple of the number of poles of the wound armature.
• the invention is applicable to a continuously rotating stepping motor it is particularly applicable to oscillating motors moving through a fraction of a revolution.
• the invention may be applied to a motor in which the permanent magnetic field device is stationary forming a stator, and the wound armature moves, forming a rotor, particularly in the case of an oscillating motor moving through only a fraction of a revolution, in which case the coils may be supplied through flexible conductors.
• the wound armature is the stator and the permanent magnet field device is the rotor.
• a feedback signal from a feedback device sensing the angular position of the rotor may be compared with a command signal from an input device to produce an error signal which is amplified to provide the motor current to move the load until the feedback signal matches the command signal. If the command signal is zero the error signal will be the feedback signal and will restore the rotor to a position (which will be termed the zero datum position) in which the error signal, and hence the torque, is zero.
• the several motor units and their feedback systems are so arranged and connected that their zero datum positions are the same but their maximum torque positions (at constant current) differ by an angle equal to the slot pitch divided by the number of units.
• Figure 1 shows a sectioned and exploded view of a hydraulic servo valve unit embodying a motor of the invention
• Figure 2 shows an exploded view of the motor unit of Figure 1
• Figures 3, 4, 5 and 6 show graphs of operating parameters of tite motor unit.
• Figures 1 and 2 show in exploded and sectioned views an electrically operated hydraulic control valve.
• the control valve is of a reciprocating spool type actuated by an electric motor embodying the invention.
• the position of the spool of the valve is detected by a transducer to give an electrical signal representative of the position so that the motor may be driven in a servo- control loop to position the valve to a desired operating position between two extremes.
• motors of the present invention may be used to drive such a control valve directly through an eccentric or crank pin mechanism without the use of intermediate gearing while still obtaining smooth and close resolution in positioning the control valve.
• the motor in Figures 1 and 2 comprises two units
• Each rotor 32, 33 incorporates samarium cobalt permanent magnets in. its outer surface and is surrounded by its corresponding stator ring carrying exciting coils 40 wound in slots 41.
• the current supplied to the motor is controlled by any convenient feedback mechanism operating in a closed loop.
• a feedback signal from a feedback device may be compared with a demand signal produced by an input device to produce an error signal which is amplified to provide the motor currents to move the load until the feedback signal matches the demand signal.
• Figure 3 is a graph showing the sinusoidal curve 10 of torque at constant current against angular movement for a single motor unit. For a four pole motor unit it will be seen that from a maximum value 11 taken as zero angle the torque remains positive from -45o to +45o. Moreover if the limits of angular movement are, say, -20o to +20o the variation is relatively small and the torque may be regarded as varying solely with current.
• the feedback signal varies with angle, for example linearly, at 15. Accordingly the actual torque if the demand were zero would change sign at a point 20 that will be referred to as the zero datum, as well as returning to zero at 21 at an angle of -45° and +45° at 22.
• Figure 5 shows the effect of the present invention. With 12 slots and two motor units the offset must be 15 and it is desirable to divide this equally so that the torque maxima are at - 71 ⁇ 2o at 26 and +71 ⁇ 2° at 27 from the zero datum position. This enables the torque of both motor units to be kept positive throughout the range from -20° to +20° from the zero datum.
• Figure 6 is a graph similar to Figure 5 showing the curves for three motor units.
• the two units of the motor are arranged so that the rotor positions at which different units would exert maximum torque (at constant current) differ by an angle equal to the slot pitch divided by the number of units, in this case half the slot pitch.
• the result is that when the instantaneous value of the cogging torque of one unit is positive that of the other unit will be equal to it, but negative so that the cogging torques will cancel out.

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• Engineering & Computer Science (AREA)
• Power Engineering (AREA)
• Permanent Magnet Type Synchronous Machine (AREA)

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Citations (7)

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• 1982
  • 1982-01-25 EP EP82900286A patent/EP0069751B1/en not_active Expired
  • 1982-01-25 DE DE8282900286T patent/DE3263064D1/de not_active Expired
  • 1982-01-25 WO PCT/GB1982/000019 patent/WO1982002619A1/en active IP Right Grant
  • 1982-01-25 JP JP57500374A patent/JPS57502248A/ja active Pending
  • 1982-09-13 US US06/422,903 patent/US4535263A/en not_active Expired - Fee Related

Patent Citations (7)

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